Many modem display devices are based on colour cathode ray tubes (colour CRTs) corresponding to the type presented above. In some advanced colour CRTs, such as the one described in WO 99/34392, the trajectories of the electron beams of the CRT are changed dynamically in order to adapt the electron beams to an increased distance between a colour-selecting electrode and the inner surface of the display. More specifically, the distance between the electron beams at the location of the deflection plane is changed as a function of the deflection of the beam across the display, i.e. as a function of the desired landing coordinates of the electron beams on the display.
However, this colour CRT, as well as many other types of CRTs, have a tendency to present variations in the purity of the white colour, i.e. deteriorated white uniformity, in the image presented on the display.